More and more offshore hydrocarbon fields are discovered in deepwater areas where there is little infrastructure and the Floating Production, Storage and Offloading (FPSO) concept can be economically competitive.
As part of concept of a FPSO for new deepwater fields, disconnectable FPSO options with focus on the vessel turret, the disconnectable system and potential riser solutions.
A typical field development which would comprise of 12 subsea wells in 6,200 ft of water, tied back to four subsea manifolds. The flow lines are for example assumed to be composed of two loops connecting to the FPSO facility via four risers.
The small amount of produced gas would be exported via a pipeline and export of the produced oil would be via shuttle tankers. The possible requirement for high pressure (and high volume) water injection was also part of the assumptions. The flow lines can be nominal 8″ pipe designed to 7.5 ksi.
Although the field would have a mud-line shut-in pressure in excess of 10 ksi, it is assumed that the design pressure of flow lines and risers can be lowered by deployment of a high integrity pressure protection system (HIPPS).
On the other hand, the potential requirement for high pressure (and high volume) water injection is needed as well, i.e. the water injection riser would have to be designed for pressures exceeding 10 ksi. The subsea architecture can be composed of two loops (with two manifolds in each loop) connecting to the FPSO facility via four risers. The small amount of produced gas can be exported via a nominal 6″ pipeline and export of the produced oil would be via shuttle tankers.
Prior to recent developments in deepwater mooring technology, the hybrid riser concept was the only solution available with disconnectable FPSOs. However, compared to SCRs or Lazy Wave SCRs, the hybrid riser concept has a more complex design, requires more hardware, requires heavy installation vessels, and is more CAPEX intensive.
In U.S. Pat. No. 5,957,074 there is shown a mooring and riser system for use with a turret moored hydrocarbon production vessel which comprises: three groups of mooring lines spaced approximately 120° apart, each group containing three individual mooring lines, the three groups of mooring lines having open sectors in-between and each being attached to the sea floor on a first end and attached to the hydrocarbon production turret on a second end; and a system to support the substantially rigid catenary riser located in the open sectors, to support the rigid catenary riser.
In the DOT 2011 paper “deepwater mooring and riser solutions for disconnectable FPSO's” published by the applicant, there is also disclosed disconnectable systems such as a Buoyant Turret Mooring (BTM) coupled with steel risers or an external turret system comprising a spar buoy which the FPSO is connected via an articulated yoke system hence decoupling the FPSO heave/pitch motions from the SCR friendly spar buoy. This type of external turret allows the steel risers and umbilicals to be in simple catenary configuration. The system comprising the BTM is provided with an internal turret FPSO supporting a disconnectable buoy (see FIG. 1). The buoy function is to support the mooring lines and risers/umbilicals upon disconnect, i.e. the buoy will slowly descend in the water column to an equilibrium condition (at least 50 m below the sea level) where there will be minimal wave kinematics.
The advantage of this concept is that all critical equipment (e.g. the swivel stack) is kept on the turret while the buoy is kept simple and its main functionality is to offer buoyancy in the disconnected scenario.
It is known to have Lazy Wave SCRs directly connected to an internal turret in a deepwater environmental (BC-10 FPSO).
A cost effective alternative is needed for hybrid risers, i.e. a turret and mooring system which would make the steel catenary riser (SCR) feasible, especially a BTM system coupled with Lazy Wave SCRs.
In connected scenarios, as the riser hang-off points move (heave, pitch and roll) with the vessel, the decoupling of the vessel motions from the riser touch down point (TDP) is achieved by utilizing distributed buoyancy in each riser and umbilical to create the “Lazy Wave” shape The system using lazy-wave SCR is more advantageous than the one using steel risers and umbilicals to be in simple catenary configuration as the riser payload on the BTM buoy when disconnected is reduced.
However, the available prior art does not mention how to ensure the integrity of the components of such systems especially after disconnection.
The system in the present invention proposes a particular disposition of the components in order to secure the integrity of the risers, umbilicals and mooring lines such that reconnection would be eased and safe with all elements in good conditions and not damaged.
The proposed system ensures that during disconnection is the relative heave motion between the buoy and the vessel and ensuring that there is no impact between the two floating bodies after the buoy separates from the turret.
Further as a quick connect and disconnect (QCDC) is provided and which can disconnect the buoy from the vessel in minutes, it is also an object of the present invention to ensure once again that even in emergency disconnection there is no damage and no impact between the risers, umbilicals and mooring lines.